feat: add MirrorNodeAccountBalanceQuery - #1814

Description:

The consensus node stops serving CryptoService/cryptoGetBalance in release 0.77, which retires
AccountBalanceQuery. Add its mirror node replacement, built to the merged
migration proposal.

• Add MirrorNodeAccountBalanceQuery, reading GET /api/v1/balances?account.id={id}
• Add MirrorNodeAccountBalance, carrying the hbar balance
• Add AccountID._MirrorNodePathID so an EVM-address alias is sent as bare hex and a public key
  alias as unpadded base32, and route the existing _MirrorNodeRequest through it
• Consolidate the mirror node REST calls onto one retry core and move FeeEstimateQuery,
  MirrorNodeContractQuery and RegisteredNodeAddressBookQuery onto it
• Add an example under examples/mirror_node_account_balance
• Fix a nil-pointer panic in PopulateAccount: it dereferenced *id.AliasEvmAddress unconditionally, so it
  panicked on any AccountID without an EVM alias.
• Skip checksum validation for IDs sent as an alias — an alias has no checksum, so
  validating one rejected the EVM-address and public-key forms SetAccountID documents
• Default the retry budget to the mirror node's 3 attempts instead of 10, capping a
  failing query at ~91s instead of ~5.7 minutes

Related issue(s):

Fixes #1792

Notes for reviewer:

Checklist

• Documented (Code comments, README, etc.)
• Tested (unit, integration, etc.)