Add validation for STX amount in signer key authorizations

[hstove]

In pox-4 we have authorizations (via pre-set authorizations or via signatures) around usage of a signer key, but those authorizations do not cover the amount of STX used.

These authorizations should include a new field, amount-ustx, which indicates the max amount of STX that can be used by a particular reward set.

Also related is #4417, which adds signer-key to stack-increase. That ticket will need to also validate that the new amount of STX locked (after the increase) does not exceed the amount-ustx included in the authorization.

[hstove]

Linking discussion: #4424

[blockstack-devops]

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