Add amt website client

[ChristopherSpelt]

Add AMT website client to fetch instruments

Implementation notes:

1. Needed to change the existing method signature of list_content of the original GitHub client so that all implementations of the method list_content abstract client have the same signature.
2. All clients are in 1 single file, to avoid 4 different small files. Ideally one would want the abstract class Client and get_client to be one 1 file, and GitHubClient in another file and GitHubPagesClient in yet another file, but this would result in circular imports.

Resolves #126

Checklist

Please check all the boxes that apply to this pull request using "x":

• I have tested the changes locally and verified that they work as expected.
• I have followed the project's coding conventions and style guidelines.
• I have rebased my branch onto the latest commit of the main branch.
• I have squashed or reorganized my commits into logical units.
• I have read, understood and agree to the Developer Certificate of Origin, which this project utilizes.

[berrydenhartog]

LGTM

[sonarqubecloud]

Quality Gate passed

Issues
1 New issue
0 Accepted issues

Measures
0 Security Hotspots
97.5% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud