feat: add interfaces to client's APIs

[bednar]

Closes #325

Proposed Changes

Added interfaces to APIs to be able to use moq library:

var mockClient = new Mock<IInfluxDBClient>();
var mockQueryApi = new Mock<IQueryApiSync>();

mockClient
    .Setup(library => library.GetQueryApiSync(null))
    .Returns(mockQueryApi.Object);

var mockTables = new List<FluxTable> { new FluxTable() };
mockQueryApi
    .Setup(api => api.QuerySync("from(...", "my-org", CancellationToken.None))
    .Returns(mockTables);

var tables = mockClient.Object.GetQueryApiSync().QuerySync("from(...", "my-org");
Assert.AreEqual(mockTables, tables);

Checklist

• CHANGELOG.md updated
• Rebased/mergeable
• A test has been added if appropriate
• dotnet test completes successfully
• Commit messages are conventional
• Sign CLA (if not already signed)

[codecov-commenter]

⚠️ Please install the to ensure uploads and comments are reliably processed by Codecov.

Codecov Report

Attention: Patch coverage is 0% with 63 lines in your changes missing coverage. Please review.

Project coverage is 87.37%. Comparing base (9bf46c1) to head (3da70af).
Report is 248 commits behind head on master.

Files with missing lines	Patch %	Lines
Client/InfluxDBClient.cs	0.00%	63 Missing ⚠️

❗ Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #327      +/-   ##
==========================================
- Coverage   88.21%   87.37%   -0.84%     
==========================================
  Files          76       76              
  Lines        6565     6628      +63     
==========================================
  Hits         5791     5791              
- Misses        774      837      +63     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.