Add helpful abstractions for common invocation types.

[Shaptic]

What

This adds four new aliases for Operation to make common versions of Operation.invokeHostFunction easier to create.

/** Returns an operation that invokes a contract function. */
function invokeContractFunction({
  contract: string,
  function: string,
  args: xdr.ScVal[],
  auth?: xdr.SorobanAuthorizationEntry[],
  source?: string
}): xdr.Operation;

/** Returns an operation that creates a custom WASM contract. */
function createCustomContract({
  address: Address,
  wasmHash: Uint8Array | Buffer
  salt?: Uint8Array | Buffer,
  auth?: xdr.SorobanAuthorizationEntry[],
  source?: string
}): xdr.Operation;

/** Returns an operation that wraps a Stellar asset into a token contract. */
function createStellarAssetContract({
  asset: Asset | string,
  auth?: xdr.SorobanAuthorizationEntry[],
  source?: string
}): xdr.Operation;

/** Returns an operation that uploads WASM for a contract. */
function uploadContractWasm({
  wasm: Uint8Array | Buffer,
  auth?: xdr.SorobanAuthorizationEntry,
  source?: string
}): xdr.Operation;

Why

Closes #700 and #697.

[github-actions]

Size Change: +30 kB (+1%)

Total Size: 3.19 MB

Filename	Size	Change
dist/stellar-base.js	2.34 MB	+22.4 kB (+1%)
dist/stellar-base.min.js	850 kB	+7.59 kB (+1%)

_{compressed-size-action}

[sreuland]

rather than imbuing base with this app knowhow, is it worthwhile for separation to implement these function wrappers in js-stellar-sdk/src/contract_spec.ts ?

[Shaptic]

I think it's still better in base because it's just a variation of the XDR rather than something related to the RPC server or Horizon. To be honest, the only reason why ContractSpec is in stellar-sdk is because it was written in TypeScript. So yeah, I do think it belongs more here.

[kalepail]

Have not reviewed the code but assuming these all work these are very helpful functions and will be a tremendous value add for a lot of folks. 👏