[R] Allow string input to as_type()

[asfimport]

Currently the function as_type() expects argument type to be a DataType, e.g.

as_type(int32())
## Int32
## int32

It would be nice if this function could also accept a string and return a DataType, e.g.  

as_type("int32")
## Int32
## int32

See discussion here: #10327 (comment)

Reporter: Ian Cook / @ianmcook

Related issues:

• [R] Work around masking of data type functions (relates to)

_{Note: This issue was originally created as ARROW-12908. Please see the migration documentation for further details.}

[asfimport]

Todd Farmer / @toddfarmer:
This issue was last updated over 90 days ago, which may be an indication it is no longer being actively worked. To better reflect the current state, the issue is being unassigned. Please feel free to re-take assignment of the issue if it is being actively worked, or if you plan to start that work soon.