Rename `CLI_PROTO_DESCRIPTOR_FILE` to `PROTO_DESCRIPTORS_FILE_PATH`

[apstndb]

Rename CLI_PROTO_DESCRIPTOR_FILE to PROTO_DESCRIPTORS_FILE_PATH

Context

Part of #47 (java-spanner compatibility). Currently CLI_PROTO_DESCRIPTOR_FILE needs to be renamed to match the upstream PROTO_DESCRIPTORS_FILE_PATH name.

Upstream References

• java-spanner (ClientSideStatements.json): PROTO_DESCRIPTORS_FILE_PATH with SET/SHOW support
• go-sql-spanner: Not present

Changes Required

Variable Rename

• Rename CLI_PROTO_DESCRIPTOR_FILE → PROTO_DESCRIPTORS_FILE_PATH
• Drop CLI_ prefix and align with java-spanner naming

Keep spanner-mycli Extensions

• Keep ADD support (append descriptor files)
• Keep .proto compilation support
• Keep HTTP URL support

Backward Compatibility

• No backward compatibility required per project philosophy

Acceptance Criteria

• SET PROTO_DESCRIPTORS_FILE_PATH = '...' works
• SHOW VARIABLE PROTO_DESCRIPTORS_FILE_PATH works
• ADD support still works
• .proto compilation still works
• HTTP URL support still works
• CLI_PROTO_DESCRIPTOR_FILE is fully removed
• make check passes