Add a pollTransaction variation to support polling after sends

[Shaptic]

What

Add a method to rpc.Server that lets you retry getTransaction based on your parameters. By default it tries 5 times with 1s of sleep between each polling attempt. However, it's flexible and you can do unhinged stuff like this:

const txStatus = await server.pollTransaction("cafebabe", {
   attempts: 100, // I'm a maniac
   sleepStrategy: rpc.LinearSleepStrategy
}); // this will take 5,050 seconds to complete

Note: I don't think this makes sense to do sending as well since it's a two-step process.

[github-actions]

Size Change: +112 kB (+0.23%)

Total Size: 49.1 MB

Filename	Size	Change
dist/stellar-sdk-minimal.js	6.76 MB	+18.8 kB (+0.28%)
dist/stellar-sdk-minimal.min.js	5.28 MB	+9.19 kB (+0.17%)
dist/stellar-sdk-no-axios.js	6.76 MB	+18.8 kB (+0.28%)
dist/stellar-sdk-no-axios.min.js	5.28 MB	+9.19 kB (+0.17%)
dist/stellar-sdk-no-eventsource.js	7.02 MB	+18.8 kB (+0.27%)
dist/stellar-sdk-no-eventsource.min.js	5.49 MB	+9.19 kB (+0.17%)
dist/stellar-sdk.js	7.02 MB	+18.8 kB (+0.27%)
dist/stellar-sdk.min.js	5.49 MB	+9.19 kB (+0.17%)

_{compressed-size-action}

[kalepail]

lgtm with the one caveat that 5 seconds is probably too short for a default. Ledgers are more often 6 seconds than 5 these days. I'd opt for a 30 second default.