Add helpers method to build authorization entries.

[Shaptic]

Replaces #632 in an attempt to tackle #575. This adds three helpers (see the jsdocs for details):

function authorizeInvocation(
  signer: Keypair,
  networkPassphrase: string,
  validUntil: number,
  invocation: xdr.SorobanAuthorizedInvocation
): xdr.SorobanAuthorizationEntry;

function buildAuthEnvelope(
  networkPassphrase: string,
  validUntil: number,
  invocation: xdr.SorobanAuthorizedInvocation
): xdr.HashIdPreimage;

function buildAuthEntry(
  envelope: xdr.HashIdPreimage,
  signature: Buffer | Uint8Array,
  publicKey: string
): xdr.SorobanAuthorizationEntry;

[github-actions[bot]]

Size Change: +46.5 kB (+2%)

Total Size: 3.07 MB

Filename	Size	Change
dist/stellar-base.js	2.25 MB	+33.7 kB (+2%)
dist/stellar-base.min.js	818 kB	+12.8 kB (+2%)

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