Update Xero integration from OAuth1 to OAuth2

[nabilfreeman]

OAuth1 has been deprecated by Xero since Feb 2020:
https://devblog.xero.com/an-update-on-why-we-are-saying-goodbye-oauth-1-0a-hello-oauth-2-0-6a839230908f

Here's a PR to support OAuth 2. Our app didn't actually work (mentioned #63) with OAuth1 because it was made in March!

I had to change the baseURL to https://api.xero.com rather than the previous https://api.xero.com/api.xro/2.0/.

Xero requires a xero-tenant-id header to be passed with subsequent requests to gain access to business accounts the user has access to.

The only way to get this xero-tenant-id is to query:

GET https://api.xero.com/connections

Then after obtaining the tenant ID you can query:

GET https://api.xero.com/api.xro/2.0/... with an added header xero-tenant-id: "abcdefg-12345".

Question: Is there an organization plan for the json files for different supported OAuth versions? E.g. xero, xero-oauth1, xero-oauth2?

[Frenchcooc]

Hi Nabil. Thanks a lot for the contribution!

Regarding the filename, as Xero has deprecated the OAuth1 integration, I totally agree with scratching the old integration (xero.json). Noone needs the OAuth1 configuration anymore. But I still don't kow what would be best to do when we would have two different authorization method that works in parallel. Using api-x-oauth1 and api-x-oauth2 seems a bit verbose, but at least it works well...