Add mmr_generateAncestryProof rpc call

[Lederstrumpf]

Description

Adds generateAncestryProof to the mmr RPC. An RPC method for generating ancestry proofs is required for cross-chain slashing by cross-chain fishermen Snowfork/snowbridge#1493.
Consequently, this PR also adds the mmr runtime api method generate_ancestry_proof. While such a method was already exposed by the beefy-mmr runtime api, this PR opts for moving it to the mmr runtime api instead due to the following considerations:

1. Invoking beefy-mmr's generate_proof method via RPC would require adding the offchain-db extension to the beefy-rpc, which is a more invasive change with boilerplate that's not needed since the mmr RPC already uses the offchain-db extension for generating leaf proofs.
2. Since the ancestry proofs are for MMR, it is more natural to expose the method directly on the mmr runtime api - the beefy-mmr pallet's generate_proof method is merely a wrapper around the mmr pallet's generate_ancestry_proof method.

Some other misc. changes documented under Review Notes.

Integration

The integration is the same as for the beefy-mmr runtime api's generate_proof method, except that the optional at specifier is removed for the method here since the method is idempotent wrt. the block height invoked at, so long as at >= best_known_block_number. Removing the specifier reduces likelihood of spurious errors from incorrect usage. I can revert the at specifier removal however if desired for compatibility.

For example use, see Snowfork/snowbridge#1493.

Review Notes

• Adds generate_ancestry_proof method to mmr runtime api (Lederstrumpf@682eb4a)
• Adds mmr_generateAncestryProof rpc method (Lederstrumpf@5d0eac9, Lederstrumpf@5d0eac9f1f)
• Adds new InvalidEquivocationProofSessionMember error to beefy pallet (Lederstrumpf@682eb4a141) (note: this change is unrelated to the PR's main purpose, but helps implementers with more granular error reporting. I'm open to removing this change).
• Deprecates pallet_beefy::generate_ancestry_proof::generate_ancestry_proof and pallet_beefy::AncestryHelper::generate_proof. Deprecation penciled in for September 2025 - I'm open to change this date or undo the deprecation. (Lederstrumpf@6619169ecd)
• Removes at specifier for pallet_mmr::generate_ancestry_proof (Lederstrumpf@bcadda2) (as mentioned in the Integration section, fine to undo)

PR can be tested using Snowfork/snowbridge#1493.