chore: asset.product typing fix

[sbrunato]

Fixes typing issues raised by mypy:

eodag_cube/api/product/_assets.py:73: error: "EOProduct" has no attribute "get_file_obj" [attr-defined]
eodag_cube/api/product/_assets.py:80: error: "EOProduct" has no attribute "rio_env" [attr-defined]

The cast tells mypy that in this package it is the eodag-cube EOProduct subclass, which does define get_file_obj and rio_env.

[github-actions]

Benchmark comparison

Compared baseline: origin/develop
Compared candidate: 5ee2ccf

--------------------------------- benchmark: 5 tests, 2 sources ---------------------------------
Name (time in us)                                     baseline Median  candidate Median   ΔMedian
-------------------------------------------------------------------------------------------------
test_benchmark_eoproduct_instantiation                        21.0615           20.6460     -2.0%
test_benchmark_eoproduct_assets_population                    41.9030           42.8750     +2.3%
test_benchmark_to_xarray_local                            10,041.8620       10,143.5990     +1.0%
test_benchmark_import_eodag_cube_subprocess               12,167.1700       12,267.2490     +0.8%
test_benchmark_import_eodag_public_api_subprocess        487,038.9150      480,962.9985     -1.2%
-------------------------------------------------------------------------------------------------

Legend:
  Cyan: reference source for comparison. Green: improvement, Red: regression.
  Δ: percentage change from reference source.