Some small improvements on Translation Providers:

• move symfony/config from dev dependency to hard dependency for all Bridges as all providers use XliffFileLoader to load pulled translations (cf. [Translation] Added Lokalise Provider #40927 (comment))
• replace all instances of

'headers' => [
    'Authorization' => 'Bearer API_TOKEN',
]

with 'auth_bearer' => 'API_TOKEN', even in tests files.

• Fix Lokalise base_uri concatenation