Add QueryAsync support for the HTTP QUERY (RFC 10008)

[scottoffen]

Summary

• Added a dedicated QueryAsync extension method family on HttpRequestBuilder, mirroring GetAsync/PostAsync/etc. with the same four overloads (no-args, CancellationToken, HttpCompletionOption, and both), for the HTTP QUERY method defined in RFC 10008. Uses HttpMethod.Query on .NET 10+ and falls back to a custom HttpMethod on earlier multi-targeted frameworks.
• Fixed FluentJsonSerializer.DefaultJsonSerializerOptions gating DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull behind #if NETSTANDARD2_1_OR_GREATER, a symbol that is only defined for the literal netstandard2.1 TFM and not for net6.0 and later. This caused null-valued properties to be serialized as "property":null instead of omitted on net6.0 through net10.0, inconsistent with the netstandard2.1 build. Changed the gate to #if !NETSTANDARD2_0, which covers netstandard2.1 and every net6.0+ TFM, including future ones added to the multi-target list.
• Added QueryAsyncTests, covering all four QueryAsync overloads and asserting the request method is HttpMethod.Query.
• Updated the root README and package README with a note describing the QueryAsync addition
• Updated docs/docs/sending-requests.md with the a QUERY section and example, and added a row to the Quick Reference table.
• Added the same QueryAsync note to docs/docs/index.md

Related issue

Fixes #61 (null properties serialized instead of omitted on net6.0+, reported against 5.0.3)

Type

• Bug fix
• Feature
• Docs
• Other: ___

Checklist

• Descriptive title and clear description
• Tests added/updated (xUnit + Shouldly + Moq if needed)
• Docs updated (if user-facing behavior changed)
• No cosmetic/whitespace-only changes
• Follows the contribution guidelines and C# conventions, with readability as a priority

Notes

• Custom verbs, including QUERY, were already sendable via the existing SendAsync(string) / SendAsync(HttpMethod) overloads. This PR doesn't add the ability to send QUERY requests; it adds a first-class, named convenience method for it, matching the DX of the other verbs.
• The QueryAsync addition is not a breaking change.
• HttpMethod.Query only exists on .NET 10+, so the string-based "QUERY" fallback is required for netstandard2.0, netstandard2.1, and net6.0 through net9.0.
• The null-serialization fix changes the wire format of outgoing JSON on net6.0 through net10.0 builds: consumers relying on the (buggy) inclusion of null properties will see a behavior change, though this brings those TFMs in line with the already-correct netstandard2.1 behavior.