Support All Data Types in All Backends

[fthaler]

Description

• Enables backend-supported int8 and int16 dtypes in frontend.
• Introduces a test to check that all data types are supported.
• Fixes full dtype support in GTC-based backends (additionally requires fix in GridTools: Fix for Integer Format Check in Python SID Adapter gridtools#1632)

Requirements

Before submitting this PR, please make sure:

• The code builds cleanly without new errors or warnings
• The code passes all the existing tests
• If this PR adds a new feature, new tests have been added to test these
  new features
• All relevant documentation has been updated or added

Additionally, if this PR contains code authored by new contributors:

• All the authors are covered by a valid contributor assignment agreement,
  signed by the employer if needed, provided to ETH Zurich
• The names of all the new contributors have been added to an updated
  version of the AUTHORS.rst file included in the PR