Expose http client or handler in constructor

[Alikont]

Hello,

AirtableBase class has internal constructor that has HttpHandler parameter.

I'd like to be able to pass my own http handler to AirtableBase in a public constructor.

• I'll be able to use it with IHttpClientFactory
• I'll be able to use Polly policies for throttling and retry

[ngocnicholas]

This is done in Airtable.net 1.2.0.