chore: load templates with explicit extensions

[jonathansamines]

Description

I am trying to use this library in the context of expo server routes, which seems to use Metro's module resolution, which does introduce special asset extensions which change the way modules are loaded for certain extensions (including xml).

All of that is to say, that when someone tries to load this library in expo, you get this error:

Metro error: Unable to resolve module ./templates/encrypted-key.tpl.xml from /Users/jonathansamines/dev/my-app/node_modules/@authenio/xml-encryption/lib/utils.js:

None of these files exist:
  * encrypted-key.tpl.xml
  * node_modules/@authenio/xml-encryption/lib/templates/encrypted-key.tpl.xml
  3 |
  4 | var templates = {
> 5 |   'encrypted-key': require('./templates/encrypted-key.tpl.xml'),
    |                             ^
  6 |   'keyinfo': require('./templates/keyinfo.tpl.xml'),
  7 | };
  8 |

I realize this library was probably not designed for environments such as expo, but I am pulling this library transitively in a way that it gets loaded but not used, so at least being able to load it would be nice.

References

N/A

Testing

All tests pass

Checklist

• I have added documentation for new/changed functionality in this PR or in auth0.com/docs
• All active GitHub checks for tests, formatting, and security are passing
• The correct base branch is being used, if not the default branch