chore: re-add BigUInt, OID imports needed when building for dfx

[ghost]

For some reason, these imports are needed when compiling agent-rs for dfx. Fixes errors of this form:

error[E0433]: failed to resolve: use of undeclared type `BigUint`
  --> /Users/ericswanson/.cargo/git/checkouts/agent-rs-6af15f3bfa206490/8fbdafa/ic-agent/src/identity/basic.rs:66:22
   |
66 |     let id_ed25519 = oid!(1, 3, 101, 112);
   |                      ^^^^^^^^^^^^^^^^^^^^ not found in this scope
   |
   = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
help: consider importing this struct
   |
1  | use simple_asn1::BigUint;
   |

This is needed for dfinity/sdk#1736