JS Client: Client Config Should Use Unique Maps Instead Of Unique Arrays

[dOrgJelli]

Is your feature request related to a problem? Please describe.
Currently the client's configuration structure uses arrays to store things like redirects, envs, plugins, etc. These arrays are "unique" based on each item's uri property.

This is not very clear from a user's perspective, and instead we should just use a unique map, where the uri is the map's key.

[dOrgJelli]

TODO: revisit this taking into account the new ClientConfig changes. #1480

[dOrgJelli]

Update: @pileks said this is ~60% done, is actively working on it, PR to come.

[dOrgJelli]

PR: #1557