Add MetadataService.getCachedStatements()

[MasterKale]

This PR adds a new getCachedStatements() method to MetadataService. This returns an array of MetadataStatement's that can e.g. be saved somewhere to later feed back into a subsequent call to MetadataService.initialize({}) as the opts.statements argument to skip a network request.

Fixes #732.

Initial run:

await MetadataService.initialize();
const statements = MetadataService.getCachedStatements();
await writeStatementsToDisk(statements);

Subsequent Run:

const savedStatements = await readStatementsFromDisk();
await MetadataService.initialize({
  mdsServers: [],
  statements: savedStatements,
});