feat: add count() method to Circuit for instruction countingFeature circuit count

[sirocco369]

Fixes #1235

This PR adds a count() method to the Circuit class that allows users to easily count instructions in a quantum circuit.

Features:

• circuit.count('gate_name') — returns count of specific gate (case-insensitive)
• circuit.count() — returns dictionary with counts of all gate types
• Handles all instruction types (gates, measurements, etc.)
• Returns 0 for empty circuits or non-existent gates

Changes:

• Added count() method to Circuit class in src/braket/circuits/circuit.py
• Added comprehensive tests in test/unit_tests/braket/circuits/test_circuit.py

Examples:

>>> circ = Circuit().h(0).cnot(0, 1).h(1)
>>> circ.count('h')
2
>>> circ.count()
{'h': 2, 'cnot': 1}

[aniksd-braket]

why has this change been made as part of this PR?