Identity servers public_key(s) base64 format needs to be clarified

[zecakeh]

Link to problem area:

C-S API:

• m.room.third_party_invite

IS API:

• POST /_matrix/identity/v2/store-invite
• GET /_matrix/identity/v2/pubkey/ephemeral/isvalid
• GET /_matrix/identity/v2/pubkey/{keyId}

Issue

The documentation of the public_key and public_keys fields issued by identity servers mention unpadded base64, the one from the IS API even linking to the definition in the appendices using standard base64.

However Sydent sends the long-term public key encoded using standard base64 and the ephemeral public key encoded as URL-safe base64 (as can be seen in matrix-org/sydent#10). So this documentation should be clarified to say that both standard and URL-safe base64 can be expected for those fields.

Related: #838.