crypto/keys: move tendermint key types to the SDK

[fedekunze]

Closes: #5819

Description

Defines Ed25519, Secp256k1, Sr25519 and Multisignature pubkey and privkeys as proto messages.

Dependency migration from Tendermint will come on another PR.

TODO:

• .proto file for types and interfaces
• function migration
• util function to return byte array instead of slice
• test key compatibility

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[lgtm-com]

This pull request introduces 2 alerts when merging 5e4724d into c36c9f1 - view on LGTM.com

new alerts:

• 1 for Unreachable statement
• 1 for Expression has no effect

[tac0turtle]

This PR may need to be broken down by keys. This PR is growing to be quite large and these are important things to get right and should be reviewed in depth.

[tac0turtle]

I believe all that is needed is to define a proto type and import the tendermint types as the used types when casting. will make the needed changes for this

[tac0turtle]

this is a simpler approach #5997 let me know if this works

[fedekunze]

Replaced by #5997