At various points we've been stuck in our TLV read/write variants
but just want to break out and write some damn code to initialize
a field and some more code to decide what to write for a TLV.

We added the write-side part of this with the `legacy` TLV
read/write variant, but its useful to also be able to specify a
function which is called on the read side.

Here we add a `custom` TLV read/write variant which calls a method
both on read and write to either decide what to write or to map a
read value (if any) to the final field.

When `OutboundPayments` calls the provided `Router` to fetch a
`Route` it passes a `RouteParameters` with a specific max-fee. Here
we validate that the `Route` returned sticks to the limits
provided, and also that it meets the MPP rules of not having any
single MPP part which can be removed while still meeting the
desired payment amount.

In some uses of LDK we need the ability to send HTLCs for only a
portion of some larger MPP payment. This allows payers to make
single payments which spend funds from multiple wallets, which may
be important for ecash wallets holding funds in multiple mints or
graduated wallets which hold funds across a trusted wallet and a
self-custodial wallet.

In order to allow for this, we need to separate the concept of the
payment amount from the onion MPP amount. Here we start this
process by adding a `total_mpp_amount_msat` field to
`RecipientOnionFields` (which is the appropriate place for a field
describing something in the recipient onion).

We currently always assert that it is equal to the existing fields,
but will relax this in the coming commit(s).

We also start including a payment preimage on probe attempts,
which appears to have been the intent of the code, but which did
not work correctly.

The bulk of the test updates were done by Claude.

In some uses of LDK we need the ability to send HTLCs for only a
portion of some larger MPP payment. This allows payers to make
single payments which spend funds from multiple wallets, which may
be important for ecash wallets holding funds in multiple mints or
graduated wallets which hold funds across a trusted wallet and a
self-custodial wallet.

In the previous commit we added a new field to
`RecipientOnionFields` to describe the total value of an MPP
payment. Here we start using this field when building onions,
dropping existing arguments to onion-building methods.

In some uses of LDK we need the ability to send HTLCs for only a
portion of some larger MPP payment. This allows payers to make
single payments which spend funds from multiple wallets, which may
be important for ecash wallets holding funds in multiple mints or
graduated wallets which hold funds across a trusted wallet and a
self-custodial wallet.

In the previous commits we moved the total-MPP-value we set in
onions from being manually passed through onion-building to passing
it via `RecipientOnionFields`. This introduced a subtle bug, though
 - payments which are retried will get a fresh
`RecipientOnionFields` built from the data in
`PendingOutboundPayment::Retryable`, losing any custom
total-MPP-value settings and causing retries to fail.

Here we fix this by storing the total-MPP-value directly in
`PendingOutboundPayment::Retryable`.

In some uses of LDK we need the ability to send HTLCs for only a
portion of some larger MPP payment. This allows payers to make
single payments which spend funds from multiple wallets, which may
be important for ecash wallets holding funds in multiple mints or
graduated wallets which hold funds across a trusted wallet and a
self-custodial wallet.

In the previous few commits we added support for making these
kinds of payments when using the payment methods which explicitly
accepted a `RecipientOnionFields`. Here we also add support for
such payments made via the `pay_for_bolt11_invoice` method,
utilizing the new `OptionalBolt11PaymentParams` to hide the
parameter from most calls.

Test mostly by Claude