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]

Codecov Report

Merging #327 (3da70af) into master (9bf46c1) will decrease coverage by 0.83%.
The diff coverage is 0.00%.

@@            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     

Impacted Files	Coverage Δ
Client.Legacy/FluxClient.cs	99.13% <ø> (ø)
Client/AuthorizationsApi.cs	87.50% <ø> (ø)
Client/BucketsApi.cs	74.12% <ø> (ø)
Client/ChecksApi.cs	100.00% <ø> (ø)
Client/DeleteApi.cs	100.00% <ø> (ø)
Client/InfluxDBClient.cs	76.01% <0.00%> (-20.56%)	⬇️
Client/InvokableScriptsApi.cs	9.09% <ø> (ø)
Client/LabelsApi.cs	96.92% <ø> (ø)
Client/NotificationEndpointsApi.cs	93.25% <ø> (ø)
Client/NotificationRulesApi.cs	91.80% <ø> (ø)
... and 10 more

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