Update at_client dependencies

Is your feature request related to a problem? Please describe.

Unit tests are failing against 3.5.0 beta SDK - https://github.com/dart-lang/sdk/issues/55911

Running dart pub outdated:

Showing outdated packages.
[*] indicates versions that are not the latest available.

Package Name       Current  Upgradable  Resolvable  Latest

direct dependencies:
archive            *3.4.9   3.6.1       3.6.1       3.6.1
asn1lib            *1.5.0   *1.5.0      1.5.3       1.5.3
cron               *0.5.1   *0.5.1      *0.5.1      0.6.0
http               *1.1.0   1.2.1       *0.13.6     1.2.1
meta               *1.11.0  1.15.0      1.15.0      1.15.0
path               *1.8.3   1.9.0       1.9.0       1.9.0
uuid               *3.0.7   *3.0.7      *3.0.7      4.4.0

dev_dependencies:
coverage           *1.6.4   *1.6.4      1.8.0       1.8.0
dart_code_metrics  *4.19.2  *4.19.2     5.7.6       5.7.6   (discontinued)
lints              *2.1.1   *2.1.1      4.0.0       4.0.0
mocktail           *0.3.0   *0.3.0      1.0.3       1.0.3
test               *1.24.3  *1.24.3     1.25.7      1.25.7

4 upgradable dependencies are locked (in pubspec.lock) to older versions.
To update these dependencies, use `dart pub upgrade`.

6  dependencies are constrained to versions that are older than a resolvable version.
To update these dependencies, edit pubspec.yaml, or run `dart pub upgrade --major-versions`.

dart_code_metrics
    Package dart_code_metrics has been discontinued. See https://dart.dev/go/package-discontinue

Describe the solution you'd like

Start with dart pub upgrade --major-versions and see where that takes us.

We need to get an understanding of how much that impacts potential users of the SDK, and where we should land with things like minimum Dart version.

Describe alternatives you've considered

We probably need to stop using dart_code_metrics and find a replacement or some other workaround.

Additional context

Two unit tests appear to fail if I run dart pub upgrade --major-versions then dart test --concurrency=1 (using Beta 3.5.0-180.3 SDK); but at least the missing snapshot problem has gone.

01:17 +293 -1: test/monitor_test.dart: Monitor constructor and start tests Monitor heartbeat sending regularly [E]
  Expected: <true>
    Actual: <false>

  package:matcher               expect
  test/monitor_test.dart 356:7  main.<fn>.<fn>

01:17 +293 -2: test/monitor_test.dart: Monitor constructor and start tests Test that exceptions encountered by Monitor when sending heartbeat request (noop verb) are caught gracefully [E]
  Expected: <1>
    Actual: <2>

  package:matcher               expect
  test/monitor_test.dart 412:7  main.<fn>.<fn>

@murali-shris please feel free to reassign to somebody else.

atsign-foundation / at_client_sdk