Allow to pass custom headers to http-transport in niljs

[ukorvl]

This pull request introduces several changes to enhance the handling of HTTP headers in the RPC client and transport layer. The most important changes include adding support for custom headers, validating headers, and updating the relevant tests.

Enhancements to HTTP headers handling:

• niljs/src/rpc/rpcClient.ts: Introduced an optional headers parameter in the RPCClientOptions type and updated the createRPCClient function to include these headers in the HTTP transport configuration. Added a call to isValidHttpHeaders to validate the headers. [1] [2]
• niljs/src/transport/HttpTransport.ts: Updated the HttpTransport class constructor to accept and pass the headers parameter to the createRPCClient function.
• niljs/src/transport/types/IHttpTransportConfig.ts: Added an optional headers property to the IHttpTransportConfig type to support custom headers in the transport configuration.

Validation and utility functions:

• niljs/src/utils/rpc.ts: Added the isValidHttpHeaders function to validate the structure and values of the provided headers. This function ensures that headers are either strings or arrays of strings.

Testing improvements:

• niljs/src/rpc/rpcClient.test.ts: Added new tests to verify the creation of an RPC client with custom headers and to ensure that invalid headers throw an error.

[KlonD90]

Valudator has expanded option of types like support of arrays, but we don't have it on typing. I think that we could omit validation or update typing here.

[KlonD90]

lgtm