feat(stake): add --relay for MEV Shield outer signer on stake commands

[claytonlin1110]

Summary

Implements --relay for btcli stake add/remove/move/transfer/swap commands to allow a separate coldkey (“relay”) to sign the outer MevShield.submit_encrypted extrinsic, while the staking coldkey continues to sign the inner staking extrinsic.

Closes #824

Motivation

Some users need operational separation:

• the staking wallet signs the actual stake call (inner extrinsic),
• while a designated relay wallet signs only the MEV Shield wrapper (outer extrinsic), so the staking wallet’s coldkey is not the outer signer.

User-facing Changes

New CLI flag

--relay <wallet name | coldkey SS58>

• `Resolves to a coldkey wallet whose coldkey is used to sign the outer MEV Shield submission.

Commands updated

• btcli stake add
• btcli stake remove
• btcli stake move
• btcli stake transfer
• btcli stake swap

Validation / Rules

• --relay requires MEV protection.
  If --relay is used with --no-mev-protection, the CLI fails with an error.
• The relay wallet is validated/resolved from the same wallet base path as the staking wallet:
  coldkey SS58: must match a coldkey present under that wallet path,
  wallet name: must exist as a wallet directory under that wallet path.

Testing

• I ran py_compile (syntax check) on the modified files to ensure there are no syntax errors.
• Full pytest tests/ could not be executed in this environment due to a missing system dependency (curses/_curses import failure during test collection).

[claytonlin1110]

@thewhaleking Hi, my first contribution here, Would you please review and comment feedback? Thanks.

[claytonlin1110]

@ibraheem-abe Sorry for pinging as i'm not sure who is the reviewer here... Thanks in advance.