Split examples & check minimum versions (#3370)

Currently, both cargo deny and our MSRV checks use Cargo.lock file which has unified features and versions across both the axum crates and all the examples. This can hide some issues as usually when someone adds an example, they might use cargo add which will silently update the dependency for the whole repository. Some compilation errors (like axum requiring [email protected] while it uses features from [email protected]) will then be hidden.

I don't think most users would ever need to use the minimal versions anyway so this is not as severe, but someone might run into compilation errors.

examples/Cargo.toml

@@ -0,0 +1,46 @@
+[workspace]
+members = ["*"]
+# Example has been deleted, but README.md remains
+exclude = ["async-graphql", "target"]
+resolver = "2"
+
+[workspace.package]
+rust-version = "1.75"
+
+[workspace.lints.rust]
+unsafe_code = "forbid"
+
+rust_2018_idioms = { level = "warn", priority = -1 }
+missing_debug_implementations = "warn"
+missing_docs = "warn"
+unreachable_pub = "warn"
+
+[workspace.lints.clippy]
+type_complexity = "allow"
+
+await_holding_lock = "warn"
+dbg_macro = "warn"
+empty_enum = "warn"
+enum_glob_use = "warn"
+exit = "warn"
+filter_map_next = "warn"
+fn_params_excessive_bools = "warn"
+if_let_mutex = "warn"
+imprecise_flops = "warn"
+inefficient_to_string = "warn"
+linkedlist = "warn"
+lossy_float_literal = "warn"
+macro_use_imports = "warn"
+match_wildcard_for_single_variants = "warn"
+mem_forget = "warn"
+needless_borrow = "warn"
+needless_continue = "warn"
+option_option = "warn"
+rest_pat_in_fully_bound_structs = "warn"
+str_to_string = "warn"
+suboptimal_flops = "warn"
+todo = "warn"
+uninlined_format_args = "warn"
+unnested_or_patterns = "warn"
+unused_self = "warn"
+verbose_file_reads = "warn"