Allow the creation of R Clients from other libraries (useful for DnD and in general)

[jcferretti]

I ran the testthat tests, they are passing.

[alexpeters1208]

We need, at minimum:

1. Test case for providing an XPtr to the Client constructor.
   a. Construct a client connection from client1 <- Client$new(target = ...)
   b. Create a simple static table with empty_table and bind to a variable
   c. Extract the external pointer with xptr <- client1$.internal_rcpp_object$.pointer
   d. Construct a client connection from client2 <- Client$new(xptr)
   e. Verify you can pull table with client2$open_table("table")
2. A test case where the first and only argument to Client$new() is not a string, not a list, and not an external pointer. Error handling as appropriate.
3. If make_table_handle_from_ticket is for external use, we need an R6-level wrapping, error handling, and (at least) negative tests as appropriate. If it is internal use, I do not see any cases of its usage.

Figure out an informative error when argument 1 to Client$new() is not acceptable, and mention that we will also accept external pointers to existing client connections.

[jcferretti]

Tests passing at this point, including new tests for arguments to initialize and make_table_handle_from_ticket.

> library(testthat)
> test_package("rdeephaven")
Loading required package: Rcpp
Loading required package: arrow

Attaching package: ‘arrow’

The following object is masked from ‘package:testthat’:

    matches

The following object is masked from ‘package:utils’:

    timestamp

Loading required package: R6
Loading required package: dplyr

Attaching package: ‘dplyr’

The following object is masked from ‘package:testthat’:

    matches

The following objects are masked from ‘package:stats’:

    filter, lag

The following objects are masked from ‘package:base’:

    intersect, setdiff, setequal, union

[ FAIL 0 | WARN 0 | SKIP 0 | PASS 302 ]
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