[Feature]: Add support for client certificate authentication for REST APIs

[petkivim]

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Summary

Client certificate authentication is a mutual certificate based authentication, where the client provides its client certificate to the server to prove its identity. Client certificate is a digital certificate which confirms to the X.509 specification.

Some REST APIs require client certificate authentication. Therefore, it should be possible to upload a client certificate to Appsmith and enable it for specific integrations. In this way, Appsmith will use the client certificate when establishing a connection to integrations as a client. Also, different integrations should support using different client certificates. A client certificate is usually protected with a password and loaded onto a client application (usually as PKCS12 files with the .p12, .pfx, .pem extension).

Why should this be worked on?

Connecting to REST APIs that require client certificate authentication is not possible without the support (or it requires an additional proxy between Appsmith and the API). For example, using Appsmith as a client for services that are available over X-Road® requires support for client certificate authentication.