Support `MuxedAccount` as an auth entry credential

[mootz12]

What problem does your feature solve?

Auth entries with a LiquidityPool, ClaimableBalance or a MuxedAccount credential type are not supported, and panic instead of throwing an error.

What would you like to see?

MuxedAccount is supported.

LiquidityPool and ClaimableBalance throw an error instead of panic.

What alternatives are there?

None

[leighmcculloch]

MuxedAccount is supported.

Muxed Accounts are not valid authorizers. (I double checked with @dmkozh offline too and he confirmed as well.)