clients/stellarcore: Add support for Core's getledgerentryraw endpoint

[SirTyson]

Stellar core recently introduced a new HTTP endpoint getledgerentryraw in the p22 release: stellar/stellar-core#4350.

Request Format

getledgerentryraw needs to be implemented into the stellarcore client as its described here. The format is a POST request as follows:

ledgerSeq=NUM&key=Base64&key=Base64...

• ledgerSeq: An optional parameter, specifying the ledger snapshot to base the query on.
  If the specified ledger in not available, a 404 error will be returned. If this parameter
  is not set, the current ledger is used.
• key: A series of Base64 encoded XDR strings specifying the LedgerKey to query.

Response Format

/getledgerentryraw

JSON response (as a TypeScript interface):

interface Response {
  ledger: number; // uint32
  entries: {
    le: string; // base64-encoded xdr.LedgerEntry
  }[]

Note that this replaces the preexisting getledgerentry endpoint, which no longer exists.

Notes

While there is no release yet with the new endpoint, stellar-core master currently has the endpoint and can be built from source. Note that the getledgerentryraw is disabled by default and must be enabled explicitly via the new HTTP_QUERY_PORT config option. See "Query Commands" section in stellar-core docs/software/commands.md.