feat(deps): make https-proxy-agent optional (#2388)

Bump https-proxy-agent to v7 and a dev dependency. This makes
configuring bundlers easier to configure and removes a direct
dependency of `https-proxy-agent`.

BREAKING CHANGE: Configuring a proxy is done by specifying the
`agent` parameter on the ConnectionOptions config. This can use be
created by libraries such as `https-proxy-agent` or any that
implements `http.Agent`.

packages/xrpl/test/webpack.config.js

@@ -95,7 +95,6 @@ function webpackForTest(testFileName) {
     resolve: {
       alias: {
         ws: './dist/npm/client/WSWrapper.js',
-        'https-proxy-agent': false,
       },
       extensions: ['.ts', '.js', '.json'],
       fallback: {